Phased loading indicator in chat shows real status from backend

Context

This is a follow-up of https://gitlab.com/gitlab-org/gitlab/-/issues/413213 & !128643 (merged)

We currently show a phased loading indicator in the chat that shows the user what is happening - e.g. which tool is being chosen:

Problem to solve

The loading sentences ("Issue Search tool is finding an answer..") are a bit technical. We could think about rephrasing them in a way that still gives a sense of progress but is more human readable. It's also a bit incongruous at the moment that we start by say "GitLab Duo is doing something" and then say "A tool is doing something"

The current structure of the sentence is [Tool] is [verb] an answer.

I propose we map each tool to its own sentence like this:

Tool	Sentence
Issue search tool	GitLab Duo is understanding the issue...
Prepare data tool	GitLab Duo is writing an answer...

That would result in a flow like this:

What user does	What chat loading state says	What's actually happening
Asks a question, e.g. summarize issue	GitLab Duo is understanding the question...	No tool has been chosen
n/a, waiting	GitLab Duo is understanding the issue...	Issue search tool has been chosen
n/a, waiting	GitLab Duo is writing an answer...	Prepare data tool has been chosen

Suggested approach

1. An engineer lists all the possible tools and their function
2. @katiemacoy and @sselhorn translate the tools to human friendly sentences
3. Engineering maps the sentences to the tools